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Ikedosoma (Annelida: Echiura: Thalassematidae) from the Tropical Pacific, with Description of a New Species

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Ikedosoma (Annelida: Echiura: Thalassematidae) from the Tropical Pacific, with Description of a New Species Species Diversity 24: 267–273 Published online 25 November 2019 DOI: 10.12782/specdiv.24.267 Ikedosoma (Annelida: Echiura: Thalassematidae) from the Tropical Pacific, with Description of a New Species Masaatsu Tanaka Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan E-mail: [email protected] (Received 21 May 2019; Accepted 12 September 2019) http://zoobank.org/46558195-1F24-4235-A01A-93C3E0D1860F A new species of the thalassematid echiuran Ikedosoma abemama sp. nov. is described based on a single specimen collected from Abemama Atoll, the Gilbert Islands, Kiribati. The species is particularly distinguishable from all congeners by the absence of the proboscis lower lip. A single specimen of Ikedosoma collected from Dam Bay, Nha Trang, Vietnam is also described, being similar to the new species, although its poor condition prevented a positive identification. The first confirmed records of Ikedosoma from the tropical Pacific Ocean are included (hitherto known only from temperate waters of Japan), in addition to a summary table of diagnostic characteristics and a dichotomous key to the species of Ikedosoma. Key Words: Spoon worm, singleton, Abemama Atoll, the Gilbert Islands, Kiribati, Nha Trang, Vietnam, Alpheus echiuro- philus, Eupontonia nudirostris. first confirmed records of Ikedosoma from the tropical Pa- Introduction cific Ocean. After long-standing taxonomic confusion, the thalasse- matid genus Ikedosoma Bock, 1942 was recently redefined Materials and Methods and its valid status confirmed. Three nominal species [I. ele- gans (Ikeda, 1904), I. gogoshimense (Ikeda, 1904), and I. qin- The holotype of the new species was fixed with Bouin’s gdaoense Li, Wang, and Zhou, 1994] have been proposed, al- fluid and subsequently transferred to 70% ethanol; the though the third is now regarded as incertae sedis (Tanaka et specimen of Ikedosoma sp. had been fixed and preserved in al. 2014). Ikedosoma is most closely related to the genus Lis- 70% ethanol (Ivan Marin, personal communication). Ob- triolobus Spengel, 1912, but can be distinguished from the servations, dissections, and drawings were made using a latter by the absence of a rectal caecum (Tanaka et al. 2014). stereoscopic microscope, and trunk musculature examined Previous records of the species of Ikedosoma have been with the aid of a light box. General terminology follows Ste- essentially limited to temperate Japanese waters (Tanaka phen and Edmonds (1972) and Tanaka et al. (2014). The et al. 2014). However, Dawydoff (1952) reported a species holotype has been deposited in the National Museum of close to I. gogoshimense [as Thalassema gogoschimense (sic)] Nature and Science, Tsukuba (NSMT), and the specimen collected from coastal waters off French Indochina (= Viet- of Ikedosoma sp. in the Natural History Museum and Insti- nam). Later, Dawydoff (1959) briefly noted the presence of tute, Chiba (CBM), Japan. For comparison, photographs of I. elegans (as T. elegans) in Indochina and Japan, possibly the live specimen of I. elegans (NSMT-Ec 113) and that of a reconsideration of his 1952 record. Furthermore, Tzeng I. gogoshimense (NSMT-Ec 116), both examined in Tanaka and Chen (1992) reported an unidentified species of Ikedo- et al. (2014), were utilized. Abbreviations: trunk length, TL; soma, being the host of a snapping shrimp, Alpheus barbatus proboscis length, PL; longitudinal muscle band, LMB. Coutière, 1897 (Anker and Dworschak 2004; Anker et al. 2005), from Taiwan. Since these reports included few mor- phological details of the specimens, the identifications re- Systematics main uncertain and should be considered as doubtful. Recently, the author had the opportunity to examine a single specimen of Ikedosoma, collected from the Gilbert Is- Family Thalassematidae Forbes and Goodsir, 1841 lands, Kiribati during “the Expedition to the North Equato- Genus Ikedosoma Bock, 1942 rial Current Areas” in 1984 (see Imajima 1986). The speci- Ikedosoma abemama sp. nov. men is described herein as a new species, and together with (Figs 1, 2A, B, 3) a Vietnamese specimen of Ikedosoma [reported as Listriolo- bus sp. by Marin (2014); and Anker et al. (2015)] of uncer- Material examined. Holotype: NSMT-Ec 187, male, tain specific identity (also described herein), constitute the intertidal sandy bottom, lagoon side of Abatiku, Abemama © 2019 The Japanese Society of Systematic Zoology 268 Masaatsu Tanaka Fig. 1. Ikedosoma abemama sp. nov. (holotype, NSMT-Ec 187). A, lateral view of the entire body, before dissection; B, dorsal view of the dissected trunk showing musculature (with aid of light box); C, magnified trunk wall showing two longitudinal muscle bands (marked by asterisks) and the continuous oblique muscle layer between them. Scales: A, B, 1 cm; C, 1 mm. Atoll, the Gilbert Islands, Kiribati (approximate geolocation: proboscis pale yellow in preservative, probably due to dis- 0°23′12.1″N, 173°47′15.4″E), collected by Teruaki Nishika- coloration by Bouin’s fluid (Fig. 1A). TL ca. 140 mm; PL ca. wa, 26 July 1984. 70 mm. Proboscis elongate, laterally curled inward, trun- Diagnosis. A species of Ikedosoma without proboscis cated at anterior extremity; dorsal surface covered with lower lip; more than two pairs of gonoducts present anterior minute papillae; lower lip absent, lateral margins not united to the ventral setae. at base; margin of mouth opening weakly undulated due to Description. Coloration in life unknown. Trunk and shrinkage (Figs 1A, 2A, B). Trunk wall thin, uniformly cov- Ikedosoma from the tropical Pacific 269 Fig. 2. Comparison of morphology around the basal proboscis among three species of Ikedosoma (ventral view). Arrowheads indicate mouth position to show lower lip absence (B) or presence (C, D). A, B, Ikedosoma abemama sp. nov., preserved specimen (holotype, NSMT- Ec 187); C, I. elegans, live specimen (non-type, NSMT-Ec 113); D, I. gogoshimense, live specimen (non-type, NSMT-Ec 116). Scales: A, C, D, 1 cm; B, 2 mm. ered with numerous papillae, particularly prominent (up to Vascular system consisting of dorsal, neurointestinal, ca. 400 µm in height) posteriorly (Figs 1A, 3). Trunk mus- ventral, and ring vessels (Fig. 3). Dorsal vessel branched, culature comprising outermost circular, middle longitudi- one branch attached to posterior end of gizzard, the other nal, and innermost continuous oblique muscle layers. LMB connected to ring vessel; branches connected by additional nine, rather inconspicuous dorsally (Fig. 1B). Paired ventral narrow vessel between distal end of former and middle part setae hook-shaped; no interbasal muscle between setal sacs of latter, forming a loop (Fig. 3A). Ring vessel incomplete- (Fig. 3A). Gonoducts filled with sperm masses, 11 in total, ly encircling posterior end of crop (Fig. 3A). Ventral vessel arranged in approximate pairs along ventral nerve cord; in running along almost entire length of ventral nerve cord, front of ventral setae, two and three gonoducts present on terminating at posterior end of postsiphonal intestine with left and right side, respectively; behind ventral setae, three mesenteries (Fig. 3). Neurointestinal vessel linked to ventral further pairs of gonoducts (Fig. 3A). All pairs of gonoducts vessel at level of first pair of gonoducts, immediately bifur- occupying ca. anterior one-fifth of trunk, with largely con- cating into a large loop, and terminating on each side of ring sistent longitudinal separation between adjacent pairs (Figs vessel (Fig. 3A). 1B, 3A). Gonostome proximal, lips elongated and spirally Etymology. The specific name is after the type locality coiled (Fig. 3A). of the new species, Abemama Atoll, and is used as a noun in Alimentary canal long, convoluted, some parts filled with apposition. fine white sand not molded into fecal pellets; foregut, intes- Remarks. The new species has the following diagnos- tine, and rectum present (Figs 1B, 3). Foregut, fastened to tic characteristics of Ikedosoma, as redefined by Tanakaet ventral trunk wall by strong sheet-like mesentery, almost al. (2014): (1) regularly thickened longitudinal muscle and straight along ventral nerve cord, and divided into pharynx, continuous oblique muscle layer in the trunk, (2) elongated, esophagus, gizzard, and crop (Fig. 3A). Intestine fastened to spirally coiled gonostomal lips, and (3) absence of a rectal trunk wall through numerous thread-like mesenteries, and caecum. divided into presiphonal, siphonal, and postsiphonal sec- The diagnostic characteristics of the species of Ikedosoma tions; ciliated groove undetectable in presiphonal section are summarized in Table 1. Ikedosoma abemama sp. nov. but present in postsiphonal section (Fig. 3). Rectum fas- differs from I. elegans (Fig. 2C) and I. gogoshimense (Fig. tened to trunk wall by several robust mesenteries; rectal cae- 2D) in the absence of a proboscis lower lip, in contrast to cum absent (Fig. 3B). Paired simple anal vesicles extending the presence of such in the latter two species [see also Ikeda laterally from rectum; ca. one-third of TL, basally attached (1904: fig. 19) for I. gogoshimense, and Ikeda (1907: fig. 4) to rectum by several robust mesenteries, entirely covered for I. elegans]. In addition, the former is unique in the genus with numerous microscopic ciliated funnels (Fig. 3B). in having at least two pairs of gonoducts in front of the ven- 270 Masaatsu Tanaka Fig. 3. Internal morphology of Ikedosoma abemama sp. nov. (holotype, NSMT-Ec 187), dorsal view. Most mesenteries and gonostomal lips,
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